Atomic and molecular physics Revision lecture

Transcription

1 Atomic and molecular physics Revision lecture

2 Answer all questions

3 Angular momentum J`2 ` J z j,m = j j+1 j,m j,m =m j,m Allowed values of mgo from j to +jin integer steps If there is no external field, energy cannot depend on the value of m i.e. energy eigenstatesare degenerate with respect to m Add two angular momenta: J` =J`1+J`2 Allowed values of jgo from j 1 j 2 to j 1 + j 2 in integer steps

4 Central field approximation H` = N i=1 - Ñ2 2m e i 2 - Ze2 4pe 0 r i + i>j e 2 4pe 0 r ij K.E. of electron i Coulomb attraction, nucleus electron i Coulomb repulsion, electron i electron j Electron-electron repulsion term is large Separate it into a central part and a residual part First neglect the residual part this is the central field approximation

5 Central field approximation Electrons moves independently in an effective central potential V(r i ) V(r i )contains the attraction to the nucleus and the central part of the electron-electron repulsion H`0 = N i=1 - Ñ2 2m e i 2 +V r i Eigenfunctions separate into products of single electron functions Separate Schrodinger equation for each electron - Ñ2 2m e i 2 +V r i y n,l,ml =E n,l y n,l,ml. Each electron labelled by quantum numbers (n,l,m l,m s ) The set of these quantum numbers is the atom configuration

6 Interactions left out of the central field approximation Residual electrostatic interaction leads to splittingsinto terms labelled by L and S Relativistic effects, most importantly the spin-orbit interaction leads to fine-structure levels labelled by J Properties of the nucleus, most importantly the magnetic moment leads to hyperfine structure levels labelled by F Interactions with applied fields, e.g. with a magnetic field leads to Zeeman splitting into sub-levels labelled by M.

7 Residual electrostatic interaction Interaction is internal to the atom total orbital and spin angular momenta are conserved Define total orbital and spin angular momentum operators: L`2 S`2 L` = l`i, S` = s`i. i i L,S,M L,M S =L L+1 L,S,M L,M S L,S,M L,M S =S S +1 L,S,M L,M S L`z L,S,M L,M S =M L L,S,M L,M S S`z L,S,M L,M S =M S L,S,M L,M S Energies depend on the values of L and S Configuration splits up into terms labelled as 2S+1 L

8 Example: Carbon 2p3p configuration

9 Interactions left out of the central field approximation Residual electrostatic interaction leads to splittingsinto terms labelled by L and S Relativistic effects, most importantly the spin-orbit interaction leads to fine-structure levels labelled by J Properties of the nucleus, most importantly the magnetic moment leads to hyperfine structure levels labelled by F Interactions with applied fields, e.g. with a magnetic field leads to Zeeman splitting into sub-levels labelled by M.

10 Spin-orbit interaction Motion of electrons through electric field produces a magnetic field Interaction of electron s magnetic moment with this magnetic field Leads to perturbation of the form: H`so = b`lsl`.s` Introduce total electronic angular momentum operator: Term is split into levels according to the value of J J` =L` +S` DE J = b 2 J J +1 -L L+1 -S S +1

11 The interval rule DE J = b 2 J J +1 -L L+1 -S S +1 DE FS =DE J -DE J-1 = b 2 J J +1 - J -1 J = bj Interval rule: Energy difference of adjacent levels proportional to the larger value of J

12 Configurations, terms, levels Example: Magnesium

13 Interactions left out of the central field approximation Residual electrostatic interaction leads to splittingsinto terms labelled by L and S Relativistic effects, most importantly the spin-orbit interaction leads to fine-structure levels labelled by J Properties of the nucleus, most importantly the magnetic moment leads to hyperfine structure levels labelled by F Interactions with applied fields, e.g. with a magnetic field leads to Zeeman splitting into sub-levels labelled by M.

14 Hyperfine structure Due to properties of the nucleus other than its charge Most important is the nuclear magnetic moment m I Leads to perturbation H`hfs =À hfs Ì.J` Introduce total angular momentum operator: ` =gi m N Ì F` =Ì+J` Splitting into hyperfine components according to the value of F DE F = A 2 F F +1 -I I +1 -J J +1

15 Example: ground state Cs DE F = A 2 F F +1 -I I +1 -J J GHz

16 Levels of structure 2S+1 L J J F F Residual electrostatic: Terms Spin-orbit interaction: Fine-structure levels Hyperfine interaction: Hyperfine levels Central electrostatic: Configurations N.B. Schematic only the number of levels depends on the details.

17 Interactions left out of the central field approximation Residual electrostatic interaction leads to splittingsinto terms labelled by L and S Relativistic effects, most importantly the spin-orbit interaction leads to fine-structure levels labelled by J Properties of the nucleus, most importantly the magnetic moment leads to hyperfine structure levels labelled by F Interactions with applied fields, e.g. with a magnetic field leads to Zeeman splitting into sub-levels labelled by M.

18 Zeeman effect (neglecting hyperfine structure) Due to the interaction of the atom with an applied magnetic field Introduces the perturbation H`Z =-m`.b Magnetic moment is sum of individual moments and is proportional to total electronic angular momentum: m` =-g J m B J` Take magnetic field along z-axis. Then: H`Z =g J m B BJ`z DE Z =g J m B BM J

19 Example: Zeeman effect in sodium

20 Selection rules for electric dipole transitions Strict rules Parity must change. J=0,±1.But not J'=0 J=0. M J =0 (parallel polarization) or ±1 (perpendicular polarization). Approximate rules l=±1 (for the electron making the transition). S=0. L=0,±1 (but 0-0 is not allowed). Good for light atoms, not so good for heavier ones

21 Spectral lines Light emitted\absorbed when atom makes a transition > spectrum Broadening of spectral lines due to: (1) Natural broadening finite lifetime of transition (2) Doppler broadening random distribution of speeds (Maxwell- Boltzmann) leads to a distribution of Doppler shifts. Usually much larger than natural broadening.